Shires quickly said she'd "jumped the gun" by discussing her idea so soon — but that didn't seem to dampen excitement about the project.

Well, fans have reason to get worked up again.

On Tuesday, Shires celebrated her birthday at Nashville's historic RCA Studio A, recording with Brandi Carlile and Maren Morris, as well as Jason Isbell, producer Dave Cobb and a slew of other musicians.

Several members of this crew documented the day on Instagram, including Carlile, who shared a photo of her, Shires and Morris with the caption "Highwomen."

By the end of the day, that word was tattooed on several participants, including Carlile and Isbell. Shires captured those moments on her Instagram story.

The "Highwomen" name, of course, is a play on The Highwaymen, the country supergroup formed in the '80s by Johnny Cash, Willie Nelson, Waylon Jennings and Kris Kristofferson. But there's reason to believe this project isn't so easily defined.

Word of "The Highwomen" first emerged during an interview Shires had with Louisville radio station WFPK. At that time, she discussed first approaching Carlile for the project.

"We were at (Nashville venue) The Basement for some reason, and I told her about this idea I had called 'The Highwomen,'" she told the station. "And she was like, 'I love that idea.' And I was like, 'I want you to be in it. And I want you to help me run this, because I'm terrible at business, and my voice isn't as great as yours.'"

Shires said she and Carlile had written two songs for the project, and according to a comment left by Morris, they were also incorporating "old folky demos" she'd made with songwriter Natalie Hemby.